1. Field of the Invention
The present invention is related to a radiation image detector that records radiation images, by generating electric charges when irradiated by radiation and accumulating the generated electric charges.
2. Description of the Related Art
Various types of radiation image detectors that record radiation images of subjects, by generating electric charges when irradiated by radiation which has passed through the subjects and accumulating the generated electric charges have been proposed and are in practical use, in the field of medicine and the like.
There are two main types of radiation image detectors. One is a direct conversion type, in which radiation is directly converted to electric charges, which are accumulated. The other is an indirect conversion type, in which radiation is converted to light by a scintillator, such as that formed by CdI:Tl, GOS (Gd2O2S:Tb), electric charges are generated by the light entering a photoconductive layer, then the generated electric charges are accumulated. There are two main types of radiation image readout methods as well. One is an optical readout method that employs readout light. The other is an electrical readout method that employs TFT'S (Thin Film Transistors).
In a radiation image detector that employs the optical readout method, electric charges are generated in a recording photoconductive layer by irradiating radiation thereon. The generated electric charges are accumulated. Linear electrodes are charged with electric charges of a polarity opposite that of the accumulated electric charges. Readout light is irradiated to generate charge pairs in a readout photoconductive layer. The electric charges of the generated charge pairs are caused to combine with the accumulated electric charges and the electric charges which are charged on the linear electrodes, to read out the accumulated electric charges.
In a radiation image detector that employs the electrical readout method, electric charges which are generated by irradiating radiation thereon are collected by pixel electrodes corresponding to each pixel of an image. The collected electric charges are accumulated in accumulating capacitors, which are connected to the pixel electrodes. The accumulated electric charges are read out by turning electrical switches, such as TFT's, ON/OFF pixel by pixel.
Japanese Unexamined Patent Publication No. 2006-156555 discloses a radiation image detector of the type that employs the electrical readout method. An insulative film, or an insulative film that includes carbon particles or metallic particles is provided to cover all pixel electrodes of this radiation image detector, in order to improve flatness and film properties.
U.S. Pat. No. 5,861,052 discloses another radiation image detector of the type that employs the electrical readout method. Semiconductors are provided to cover the edges of individual pixel electrodes of this radiation image detector, in order to prevent movement of electric charges among adjacent pixel electrodes.
The aforementioned linear electrodes and pixel electrodes are detecting electrodes for detecting signals corresponding to electric charges which are generated by irradiating radiation onto photoconductive layers. Electrical fields are likely to be concentrated at the edges of detecting electrodes, and therefore, it is likely for electric charges to be injected into these edges. If electric charges are injected into detecting electrodes, image faults occur, and image quality deteriorates. Covering detecting electrodes with insulative materials is an effective measure to prevent electric charge injection. Japanese Unexamined Patent Publication No. 2006-156555 discloses a configuration in which the pixel electrodes are covered by the insulative film. However, the insulative film covers the flat electrode portions of the pixel electrodes, in addition to the edges thereof. Therefore, the charge transport properties are poor, resulting in reduced sensitivity and poor residual image lag characteristics. Note that Japanese Unexamined Patent Publication No. 2006-156555 proposes covering the pixel electrodes with a film that includes particles having charge transport properties in order to improve conductivity. In this case, however, electric charges will be more likely to be injected into the electrodes, thereby causing image faults.
U.S. Pat. No. 5,861,052 discloses a configuration in which the edges of pixel electrodes are covered by semiconductors. Semiconductors are highly conductive, which is advantageous from the viewpoints of sensitivity and residual image characteristics. However, the amount of injected charges will increase, and therefore this configuration is not an effective method for reducing image faults.